The present invention relates to flowmeters of the type wherein a fluidic oscillator oscillates at a frequency which is proportional to volumetric flow.
The prior art includes a number of flow sensing devices based on oscillatory flow effects. These devices take the form of flowmeters generating a frequency signal which is, more or less, proportional to volumetric flow. Some of these devices have shown considerable success in general and special applications in the face of strong competition from well refined and long established conventional instruments due to inherent advantageous properties, such as those given by few or no moving parts and simplicity of construction, insensitivity to adverse environments and liquids, high accuracy, and low cost.
From the point of view of their operating mechanisms, these novel flowmeters fall between positive displacement meters and differential pressure devices. Not unlike turbine flowmeters, these sensors rely fundamentally on the quantization of flow into discrete volumes, albeit not isolated from each other as in positive displacement volumeters. Consequently, some advantages similar to those of turbine meters are offered, whilst disadvantages of moving parts are reduced or avoided. In general also, comparatively lower costs are characteristic of these devices.
Examples of these prior art flowmeters are found in U.S. Pat. No. 3,589,185 (Burgess) and 3,690,171 (Tippetts et al). In the Burgess flowmeter an obstruction is placed in a flow channel to shed a vortex street downstream of the obstruction. The frequency of the vortices is proportional to the flow through the channel and this frequency is transduced to an electrical or other signal to provide a measure of the flow. The Tippetts et al device employs a fluidic oscillator in which the flow is passed through a nozzle to form a jet which is caused to deflect in an oscillatory manner transversely of the jet flow direction. The oscillation frequency is proportional to the flow and is sensed acoustically.
The vortex-shedding approach of Burgess is very sensitive to noise and its operating range is curtailed by certain limiting flow criteria. The Tippett et al approach uses built-in fluidic amplification to increase the gain and minimize signal-to-noise problems; however, oscillation in the Tippetts et al device does not occur at low flow rates so that the device cannot be used to measure low flow rates. In addition, both Burgess and Tippets et al introduce flow impedance and attendent losses into the measured flow.
It is an object of the present invention to provide a low impedance fluidic oscillator flowmeter which is relatively insensitive to noise and which is useful at even extremely low flow rates.